Myelin is a layer of insulation that covers neuronal projections called axons in the vertebrate nervous system. In the peripheral nervous system, specialized cells called Schwann cells spiral themselves around axons to form the myelin sheath. Myelin ensures that nerve impulses travel quickly and efficiently, ultimately allowing for the entire nervous system to function properly. Disruptions to the myelin sheath in disease (like multiple sclerosis or peripheral neuropathy) or after injury (like spinal cord trauma) lead to devastating symptoms, significant morbidity, and myelin loss can lead to permanent neuron loss, and ultimately, paralysis. Currently, no treatments exist to prevent demyelination or to hasten remyelination, and there is therefore a pressing need to develop therapies that address these issues. To this end, we must learn more about the mechanisms that govern myelination, myelin maintenance, and remyelination. We discovered that the orphan G protein-coupled receptor, Gpr126, is an essential component of the incompletely understood axon-Schwann cell signaling nexus that controls myelination. In Gpr126 mutant mice, Schwann cells associate with axons, but fail to spiral their membranes to generate the myelin sheath. G protein-coupled receptors are excellent drug targets, representing at least one-third of all approved drugs; thus, Gpr126 represents an extremely attractive potential target to stimulate remyelination in humans with myelin disease or injury. We therefore propose to dissect the mechanisms by which Gpr126 controls myelination and to determine if Gpr126 is required for myelin homeostasis or remyelination after injury. These studies encompass our broad goals to define the mechanisms that form, maintain, and regenerate myelinated axons in the vertebrate nervous system. In the first aim, we will define the signaling pathway downstream of Gpr126 activation by performing biochemical analyses on Gpr126 mutant tissue and cells. In the second aim, we will test the hypothesis that Gpr126 is required autonomously in Schwann cells for myelination. We will employ conditional mouse mutants to delete Gpr126 specifically in Schwann cells or in neurons, and we will perform immunohistochemical and ultrastructural analyses to determine the consequences of cell type specific loss. In this aim, we will also define the developmental window of Gpr126 requirement by temporally deleting Gpr126 in developing embryonic and perinatal transgenic mice. In the third aim, we will define the roles of Gpr126 in adult peripheral nerve. Specifically, we will determine if Gpr126 is required for myelin maintenance, demyelination, and/or remyelination after nerve injury by temporally deleting Gpr126 in mature nerve and in injured mature nerve. Together, these experiments will define the mechanisms by which Gpr126 controls myelination, will elucidate if Gpr126 is required for remyelination in adult nerve, and may lay the foundation for future therapeutics that stimulate myelin repair in humans.